Effects of warming seasonal rotational grazing on plant communities' structure and diversity in desert steppe

Abstract Grazing is the basic way of grassland utilization, and reasonable grazing is an important way to maintain the health of the grassland ecosystem. However, the traditional grazing time in warming seasons is negative for sustainable desert steppe ecosystem. Determining reasonable grassland grazing methods is to remain a critical issue for the ecological conservation and rational utilization of desert steppe. Therefore, our objectives were to explore the effects of warming seasonal rotation grazing on the species diversity and functional diversity of grassland plants and to reveal controlling factors of plant community diversity. The warm‐season rotational grazing modes included traditional time of grazing (FG), delayed start of grazing (YG), early end of grazing (TG), delayed start early end of grazing (YT), and enclosed steppe (CK). The results showed that the important value of Agropyron mongolicum of the gramineae and Lespedeza potaninii of the leguminosae in YG increased by 12.10%–120.66% and 23.57%–34.25% than other treatments (CK, FG, TG, and YT), respectively. Therefore, the YG treatment has more advantages on the IV of A. mongolicum of the gramineae and L. potaninii of the leguminosae. Warming seasonal rotational grazing (FG, YG, TG, and YT) significantly increased the important value of Leymus secalinus by 51.43%–79.64% compared with CK (p < .05). Compared with CK, FG and YG promoted the growth of gramineae and appropriately reduced the proportion of forbs. There was no significant difference in the Shannon–Wiener index between grazing treatments and CK, while the Shannon–Wiener index in YT increased by 21.43% and 15.71% compared with FG and YG (p < .05). The functional richness index in FG and YG significantly decreased by 19.05%–23.81% compared with CK and TG (p < .05). The results of the redundancy analysis showed that the diversity of plant communities was mainly affected by soil‐available nitrogen. These observations indicated that delayed start of grazing can improve the diversity of plant communities by increasing the important value of dominant plants in the community and promoting the growth of gramineous and leguminous plants, thereby optimizing the composition of community structure. Our findings can provide a theoretical basis for formulating a reasonable and scientific grazing period in desert steppe.


| INTRODUC TI ON
Grassland ecosystem is the largest terrestrial ecosystem and an essential component of the global natural ecosystems (Tian et al., 2020). Grassland plays a vital role in climate regulation, nutrient cycling, and soil and water conservation (Chadaeva et al., 2021;Villoslada Peciña et al., 2019). Moreover, natural grassland is also the critical material support for developing grassland animal husbandry (Amidzic et al., 2020). Therefore, grazing is the primary disturbance factor in natural steppe systems. Livestock has essential effects on the structure and function of plant communities through feeding, trampling, and excretion (Benjamin et al., 2014;Schönbach et al., 2011;Shan et al., 2011), and even affects the succession process and direction of communities (Guo et al., 2021). However, longterm and unreasonable grazing methods will directly change the morphological structure, productivity, and grass seed structure of the steppe Zhang, Wang, et al., 2018) and then affected the soil structure and nutrient cycle of the steppe and the production performance of livestock (Boval & Dixon, 2012), resulting in significant reduction in steppe regeneration capacity, biomass and nutrient content, which eventually led to steppe degradation (Peco et al., 2017). Therefore, the utilization of the grassland in a reasonable way of grazing is a hot topic, and it has received more and more attention from scholars.
Currently, many researchers focused on how to use grassland resources with reasonable grazing methods, including the impact of different grazing intensities and different grazing methods on grassland productivity, species composition, vegetation diversity, and soil properties. Research showed that aboveground biomass is often increased by intermediate grazing compared to that by light and heavy grazing, due to the fact that medium grazing resulted in a higher species diversity than no or low grazing (Gong et al., 2014;Yan et al., 2013). Grazing intensity can change plant community structure, soil microenvironment, and soil microbial diversity and activity (Stavi et al., 2008). Wang et al. (2019) reported that compared with traditional free grazing, rotational grazing can significantly improve the productivity, stability, resilience, and other system functions of the grassland ecosystem; the standing stock and productivity of dominant species have been improved. Under moderate grazing intensity, warm-season grazing increased forb functional group proportion, plant density, and evenness index, and it was more conducive to the maintenance of species diversity and evenness of steppe community compared with cold-season grazing . However, cold-season grazing was conducive to the accumulation of aboveground biomass of plant community in desert steppe (Tian et al., 2020). Cox et al. (1989) considered that the best grazing practices were to graze in spring but not in autumn as green grass is of the best quality in spring, while yields and quality decline rapidly due to dried forage in autumn. Wu et al. (2017) also found that both summer and winter grazing reduced the proportion of gramineous plant functional groups and increased the proportion of forbs plant functional groups. However, cold-season grazing was conducive to the accumulation of aboveground biomass in steppe community (Tian et al., 2020). Nie and Zollinger (2012) reported that compared with continuous grazing, delayed grazing in spring and autumn increased the tiller density of perennial grass, moreover, the ground coverage increased by 27% on average after delayed grazing in autumn. In general, different grazing regimes had different effects on plant composition. Seasonal grazing provided a specific period for plant recovery in grassland, which might increase resistance to grazing and alter the development of plant composition, especially in heavy grazing (Sternberg et al., 2015). Wu et al. (2017) showed that seasonal grazing mainly affected soil bulk density, water content, and distribution of soil carbon, nitrogen, phosphorus, and other nutrients through livestock activities, and ultimately affected plant community composition and species diversity. Therefore, grazing regimes (seasonal grazing) are essential for grassland management.
Recent interests in the effects of grazing on plant functional traits and functional diversity, however, further research is needed to explore the relationships between warming seasonal grazing and plant community composition and functional groups, and environmental factors controlling vegetation diversity under different warming seasonal grazing regimes.
Chinese desert steppe covers an area of 1.73 × 10 8 ha, accounting for 22.70% of the national territory, and plays essential roles in livestock production and ecosystem protection . The area is a typical semi-arid region with a drought climate and is also a fragile habitat for animals and plants. Therefore, the sustainable utilization and management of desert steppe is always an issue of concern to scholars. Previously reported that rational grazing of grassland was beneficial to the stability of grassland vegetation community, the increase in species diversity, and the maintenance of the multifunction of the grassland ecosystem. However, the effects of warming seasonal rotational grazing on the composition and Our findings can provide a theoretical basis for formulating a reasonable and scientific grazing period in desert steppe.

K E Y W O R D S
desert steppe, functional groups, plant community characteristics, species diversity, warming seasonal rotational grazing

T A X O N O M Y C L A S S I F I C A T I O N
Applied ecology diversity of plant community in desert steppe need to be further explored. Therefore, in our study four warming seasonal rotational grazing methods were conducted in desert steppe, including the enclosed steppe (CK), traditional time of grazing (FG), early end of grazing (TG), delayed start of grazing (YG), and delayed start early end of grazing (YT). The main objectives of our study were to: (1) explore the impacts of different grazing times on the composition of func-

| Experimental design
The grazing experiment was conducted in a 15a enclosed desert steppe from May to October in 2018 and 2019. It involved five treatments with a completely randomized design, including traditional time of grazing (FG), delayed start of grazing (YG), early end of grazing (TG), delayed start and early end of grazing (YT), and the control enclosed grassland (CK-enclosed steppe). In total, 18 plots were sampled, covering an area of 4.27 ha for each. The grazing intensity was determined to be 0.75 sheep·units ha −2 , and the grazing mode was rotation grazing in four areas (Wang et al., 2019;Yang et al., 2006). The grazing period is 36 d. In each rotation grazing period, the grazing days of the plot are 9 d and the grazing season is

| Measurements of plant diversity and soil properties
The vegetation community characteristics and soil properties under different treatments were investigated in September 2019. Ten subplots (1 m × 1 m for each) were randomly conducted in each treated plot. The species of each subplot were recorded during the monitoring period. Plant coverage, height, frequency, and density were also recorded in each subplot (Shen et al., 2021;Zhou et al., 2020).
And all species were divided into three classes (gramineous grasses, F I G U R E 1 Location of study area leguminous grasses, forbs grasses) into the subplots, and then col- where RC is relative coverage; RH is relative height, and RB is relative biomass.

Species diversity
Four commonly used species diversity indexes were selected in this study, namely R, H, J, and D. The calculation formula is as follows (Jäschke et al., 2020): where S is the number of species, P i is the proportion of individuals of species i in the quadrat, n i is the biomass of species i in the quadrat, and N is the total biomass of all species in the quadratic.

Functional richness
It is obtained by calculating the area or volume of the smallest polygon in the trait space, expressed as a functional richness index (FRic), which is used to measure the size of the ecological niche occupied by species, reflecting the stability and anti-disturbance ability of the community (Mason et al., 2005).
where SFic is the niche space filled by the species within the community, and Rc is the absolute range of the character (Mason et al., 2005).

Functional evenness
The functional evenness index (FEve) is a measure of the distribution law of species' functional traits in the occupied trait space and is used to predict resource utilization (Song et al., 2011). Its calculation formula is: where S is the number of species, PEW l is the weight of branch length, EW l is the branch length, l is the branch length, and W i and W j are the relative abundances of species, respectively.

Functional divergence
The multidimensional functional divergence is expressed by the functional divergence index (FDiv), which measures the maximum divergence of the abundance distribution of community functional traits in the trait space. It is used to predict the dispersal of (1) resources (Jäschke et al., 2020). The specific calculation formula is as follows: where Δd is the dispersion degree weighted by abundance, and dG is the average distance between species i and the center of gravity.

| Statistical analysis
Descriptive statistics were conducted for the plant diversity index  There were no significant differences in Fric index among TG, YT, and CK the same pattern of the Fric index occurred in FG, YG, and YT (p > .05, Figure 4)

| Effect of soil properties on plant community diversity
The results of redundancy analysis showed that the eigenvalues of the first two sorting axes were 0.5643 and 0.0145, respectively. The first axis explained 56.43% of the relationship between explanatory and response variables, and the second axis explained 1.45%. The correlation coefficients between environmental factors and community diversity in the first two axes were 0.7663 and 0.8484 (Table 3) indicated that soil alkali hydrolyzed nitrogen was the largest factor to explain plant community changes, with an explanatory amount of 34.0% and a contribution rate of 57.92% (Table 4). Therefore, under grazing disturbance in the study area, soil available nitrogen is the dominant factor of plant community species diversity change ( Figure 5).

| Warming seasonal grazing on vegetation community characteristics
Grazing is the primary disturbance factor of the natural steppe system. It mainly affects the steppe community structure and species composition by regulating the community species and functional diversity (Fedrigo et al., 2017). Among, the animal feeding and trampling directly affect the structure and composition of the steppe plant community and then affect the proportion of plant species and quantity in the community (Kraaij & Milton, 2006). In this study, short-term warming seasonal had no significant effects on the composition of the dominant plants in desert steppe, which mainly in- and biennial herbs (Yu et al., 2021). Therefore, short-term rotational grazing had no significant effect on dominant species. In different grazing plots, the importance value of gramineae increased significantly, while the significance of leguminosae decreased significantly.
A better explanation is that the grazing tolerance of leguminous forages is lower than that of gramineae (Zhang, Dong, et al., 2018), and better palatability of leguminosae (Pulido et al., 2016). Our results showed that enclosed were not conducive to the growth of gramine-  Note: AN, SP, SMC, TN, AK, and OC represent available nitrogen, soil porosity, soil moisture content, total nitrogen, available potassium, and organic carbon.

TA B L E 4
The correlation coefficients, explanatory quantity, and contribution rate of environment variable and ordination axes of redundancy analysis (RDA) fine pasture grass; reducing the disturbance of livestock to steppe vegetation and the occupation of toxic weeds can also promote the optimization of community structure (Luo et al., 2014).

| Warm-seasonal grazing on vegetation diversity
Species diversity represents a dimension of biodiversity, functional diversity is based on other dimensions of biodiversity, and the combination of species diversity can better explain changes in community composition, structure, and function. Community species diversity plays an essential role in maintaining the stability of ecosystem structure and function in desert steppe (Chao et al., 2014).
Animal feeding is one of the critical factors controlling species diversity (Liu et al., 2015). Grazing disturbance leads to changes in plant community stability, plant community structure, and species diversity. The results of this experiment showed that short-term seasonal grazing mainly affects the species richness index in desert steppe. Due to higher grazing frequency resulting in lower species richness index, reducing the frequency of grazing disturbance was more beneficial to maintain higher species richness. With the livestock feeding frequency increasing, some species in the community disappeared, and the richness index of the community decreased (Dorrough et al., 2007). In our study, the Patrick index of YT is higher than that of CK, indicating that lower grazing frequency is conducive to the improvement of species richness, and the same result was reported by Zhang, Dong, et al. (2018) who considered that with the grazing intensity increasing, the species diversity of the community decreased significantly. The increase in livestock feeding frequency will lead to the disappearance of some species in the community and reduce the richness index of the community. The Shanon-Wiener index of YT (delayed start and early end of grazing) was the highest, this result indicated that low-frequency grazing in warm season can promote the increase in the Shanon-Wiener index, a similar result was obtained by Karami et al. (2019). In the sample plots with low grazing frequency, livestock feeding weakens the competitiveness of dominant species, provides opportunities for the invasion of some species, and increases the species diversity of the community. The feeding of livestock will also affect the sexual reproduction of plants and change the community structure (Han & Ritchie, 1998). Species diversity refers to the species-specific variability of biodiversity. Functional diversity refers to the overall differences in functional traits among species in the community, controlling by the interaction between species and environmental factors. Functional and species diversity are critical predictors of ecosystem function (Huang et al., 2019). To reveal the relationship between functional diversity and ecosystem function, functional diversity is often defined as functional richness, functional evenness, and functional divergence (Mouchet et al., 2010). Our results showed that the community functional richness index was more sensitive to grazing, and grazing could reduce the community functional richness index in desert steppe. The main reason is that grazing can weaken the competitive advantage of dominant species, change the niche of species in steppe, cause fierce species competition, and reduce the functional richness index (Schultz et al., 2011). Delayed grazing had a significant impact on the later growth of vegetation and reduces the functional richness of the community. In the grazing plots that ended in advance, the functional richness index is high, which means that the niche of each species in the community is fully occupied.
The superposition of various characteristics of species increases the richness of functional traits in the community, which can more effectively buffer the interference of the external environment, increase the anti-interference ability, and the community also has higher stability (Tilman, 1996).

| Relationship between plant community diversity and soil environmental factors
Grazing is one of the important reasons that affect grassland di- and FRic index. The red arrow represents the ecological factors (explanatory variables), the blue arrow represents the plant diversity index (corresponding variables), the cosine value of the angle between the hands represents the correlation between the two, and the length of the explanatory variable needle represents the degree of interpretation of the response variable, and the longer the needle, the stronger the degree of understanding. of communities (Niu et al., 2015). At the same time, the comprehensive effects of livestock trampling, excretion, and other activities will also change the soil properties (Deng et al., 2017). Shannon-Wiener, Simpson index in sandy grassland. A better explanation is that nitrogen is the primary limiting resource of plant productivity and the main factor affecting the growth of grassland plants (Li et al., 2010). In the study, YG has the highest available nitrogen content (p > .05) (

| CON CLUS ION
Short-term warming seasonal rotational grazing had no effect on the

CO N FLI C T O F I NTE R E S T
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available YT 5.75 ± 0.35ab 56.03 ± 0.37a 9.65 ± 0.25ab 0.26 ± 0.01ab 53.10 ± 3.65ab 6.32 ± 0.85b Note: CK-enclosed steppe; FG-traditional time of grazing; TG-early end of grazing; YG-delayed start of grazing; YT-start late and end early of grazing. AN, SP, SMC, TN, AK, and OC represent available nitrogen, soil porosity, soil moisture content, total nitrogen, available potassium, and organic carbon. a, ab, b indicated significant differences among different treatments at p < .05.